I've spent more time testing and I'm afraid the issue persists. Setting IPv6 to local did not resolve the issue as I had first thought. Time and again, my TC will grey out in Airport Utility and it says 'device not found.' Only turning wifi off and back on, or rebooting the TC, will make it visible again. sometimes it will be visible for hours, other times just minutes. Occasionally the TC has become visible again all by itself, without me doing anything. But this is rare. What bothers me most about this is the complete randomness of this problem!
Having read other threads I tried the following, none of which made a difference:
- create a separate name for the 5GHz network
- reboot my Netgear router which I'm using in modem mode
I've also observed various strange things in the Internet settings as seen in Airport Utility (as stated above my internet connectivity and wifi on my imac and all other devices is never interrupted):
- the router address sometimes changes from 77.101.152.1 to 10.0.1.1 (this happens when the TC disappears
from Airport Uitlity, but not every time)
- the first DNS server changes from 194.168.4.100 to 10.0.1.1 (as above)
- the second DNS server never changes. It is 194.168.8.100
When the TC is visible in Airport Utility (after turning wifi off/on), the names of the wireless clients connected to it sometimes change. For example, my imac is usually displayed by its name 'Janimac' but sometimes it displays its wifi address instead (D8:30:62:66:24:38). The same thing sometimes happens with my iphone.
As described above, this issue only occurs in Airport Utility on my imac. Using Aiport Utility on my ipad or iphone shows everything working fine at all times. Unlike people in other threads I've read, I never lose my wifi internet connection on any device.
TimeMachine is occasionally not backing up when it says it will (in TM preferences). But when I ask it to manually "back up now" it always works, even when the TC is greyed out in Airport Utility. So TM has no problem finding the TC disk even though Airport Utility can't see it.
Mounting the TC disk/sparsebundle manually (by double-clicking) so that Time Machine Backups appears in Finder is also no problem. I have also noticed that while a TM backup is running, the drive will mount automatically (I can see this in Disk Utility). After the TM back up is finished, it will unmount again automatically. I'm assuming that's how it should be.
I'm reluctant to try many more manual changes as I don't believe it will help and the problem seems to be one of Airport Utility on my imac, not of the TC which is working fine, both as a router and back up drive. I will try changing my DHCP lease from 1 day to 1 hour as you suggest but I've read elsewhere that people have tried this and/or setting it to 30 days with no improvement. But I'll give it a try. I"m less sure about forcing to AFP protocol. It just shouldn't be that complicated.
I'm tempted to just ignore Airport Utility on my imac as everything seems to be working even though it's telling me it can't find the TC. But because my TC and network setup is all new, I'm still concerned something might not be right.
You say lots of people are having this issue, esp since Maverick, but I've done lots of searches and only found similar, not identical issues. Other people's issues seemed to be worse with intermittent wifi connection.
I will keep trying but grateful for any more advice you might have. Also wondering if someone else has a view e.g. Bob Timmons seems to have contributed to similar discussions.
Thanks!